Experiments on a new cathode dark space in helium and hydrogen

In a paper recently read before this Society, the author described some experiments on the length of the Crookes dark space under varying conditions in different gases. While the gas hydrogen was under this investigation, a slight inequality of light was noticed very close to the cathode, which, under the prevailing conditions, was not definite enough to warrant further attention at the time. When, however, helium was introduced into the apparatus, this phenomenon became strikingly clear, showing itself to be a hitherto undescribed dark space very close against the cathode and inside the Crookes dark space, possessing very different properties from the latter. By the time the measurements of the Crookes dark space in helium were completed, the behaviour of the new dark space had suggested a simple explanation, which led to a series of experiments, of which this paper is a description.Apparatus . —The preliminary observations of the new dark space were made in the “guard ring cathode” discharge tube already described, but when it was realised that an accurate knowledge of the current density was not required, this was replaced by a smaller cylindrical tube, 8 cm. in diameter, with more uniform walls and plane aluminium cathode and anode. With this exception, the apparatus was entirely as described in the previous paper.